Field of the Invention
The present invention is related to the field of underwater pelletizers and, more particularly, to an extrusion die plate for an underwater pelletizer having a die plate body with an apertured downstream face and a solid face plate.
Description of the Related Art
Known underwater pelletizers include an extrusion die or die plate having extrusion orifices extending therethrough and through which molten polymer is extruded. A cutter hub with cutter knives is oriented in rotatable facing relationship to the face of the extrusion die plate to cut the strands of polymer into pellets. A water box or cutting chamber with a water inlet and outlet provides circulation of water to cool and solidify the strands of extruded polymer to enable the knives on the cutter hub to cut the strands into pellets. The water circulating through the water box also entrains the pellets into a slurry of water and pellets which are discharged through the outlet of the water box.
A known practice is to provide a recess in the die face of the extrusion die plate and place insulation material in the recess to reduce heat transfer from the extrusion die plate and molten polymer being extruded through the extrusion orifices to the water circulating through the water box. The insulation material inserted into the recess is typically a gasket material mounted in place by a suitable mastic, glue or the like and covered with a thin metal plate in the recess. The thin plate is secured to the die plate by suitable fasteners to maintain the insulation material in place in the recess. The insulation material is subjected to substantial degradation due to the temperatures encountered in the recess and, in some instances, does not effectively insulate the central recess area of the die face. Ineffective insulation can result in excessive cooling of the molten polymer as it is being extruded through the extrusion orifices causing freeze off of the molten polymer at the die face.
One solution to these problems is to provide an insulation plug in the form of a circular plate that substantially fills the recess in the die face, as set forth in U.S. Pat. No. 6,824,371 (“the '371 patent”), which is commonly owned by the assignee of the instant application. The entire disclosure of the '371 patent is hereby expressly incorporated by reference in its entirety as though fully set forth herein.
The circular plate of the '371 patent, referred to herein as a gasket eliminator plug (GEP), reduces the transfer of heat from the molten polymer being extruded through the orifices in the extrusion die plate to the water circulating through the water box of the underwater pelletizer, and further is not subject to deterioration and degradation as encountered with conventional insulation techniques. However, the tolerance between the GEP of the '371 patent, or conventional insulation materials, and the die face is very small. Hence, it is easy to damage the die face when installing or removing the GEP during routine maintenance. This is a problem in that the GEP is relatively expensive to manufacture and yet, to work correctly, must have a tight fit within the die plate.
Another solution to the problem of excessive cooling of the molten polymer as it is being extruded through the extrusion orifices is described in U.S. Pat. No. 8,512,021 (“the '021 patent”), which is also co-owned by the assignee of the instant application. The entire disclosure of the '021 patent is hereby expressly incorporated by reference in its entirety as though fully set forth herein.
The downstream face of the solid face die plate described in the '021 patent has a solid center that eliminates the need for the GEP. The solid face die plate works well when palletizing polymers like polyethylene, EVA and PVC. However, when pelletizing high temperature materials like PET, Nylon 6,6 and polycarbonate, it would be desirable to have even better thermal performance than that provided by the solid face die plate design of the '021 patent.